Added ExectionCommand

ExectionCommand is a class that stores all data related to an execution command like:

- Input parameters like executable and arguments to be used to run the shell command, etc
- Output parameters like stdout, stderr and exitCode.
- Error info generated internally by termux outside the shell in errCode and errmsg.
- Command info like, id, label, description, help info, etc.
- Other config info like for how termux should handle the command.
- The pending intent if any that should be sent after execution to command requester.
- The help for the plugin API that was used to send the intent.
- Current and previous state of the command.

This allow easier management and passing of execution command data between classes and management of it. This will later allow each ExectionCommand command to be linked to a Terminal Session, to handle post processing and failure management.
The ExectionCommand also provides functions to get its data in markdown format, which can be used by failure or success reports generated for the command that are shown to the user. The commandHelp and pluginAPIHelp can also be specially useful to provide info to users on how to manage failures that are generated.